This application claims German Application No. 101 28 219.2, filed Jun. 11, 2001, the complete disclosure of which is hereby incorporated by reference.
a) Field of the Invention
The invention is directed to topographic measurement of eye structures such as the cornea and lens in ophthalmology.
b) Description of the Related Art
As the result of new developments in ophthalmology which are characterized by many different types of surgical procedures on the lens of the eye (e.g., cataract surgery) and on the cornea (refractive cornea surgery), there is a considerable demand for measurement methods which quantify the entire structure of the eye topographically. While several methods known under the heading of ray tracing can determine the modulation transfer function and accordingly also the point spread function of the eye [for example, R. Navarro, E. Moreno, C. Dorronsoro, J. Opt. Soc. Am., Vol. 15 (1998): 2521-2529], these methods only measure the total effect of all optical components of the eye and do not provide any information about the influences of the individual components of the eye and particularly about the exact geometry of these components. However, in order to analyze what ophthalmologic procedure has what effect on the eye or, conversely, what influence is exerted by what optic component of the eye, it is necessary to know the exact geometry of all optically active components. For this purpose, the topography of the intraocular boundary surfaces must be measured.
Coherence topograms, described in A. F. Fercher and C. K. Hitzenberger in Springer Series in Optical Sciences (ed. T. Asakura), Vol. 4, Springer Verlag, Berlin 1999, are a suitable optical method for this purpose. Optical coherence topograms are obtained from a series of z-signals measured in longitudinal direction by short coherence interferometry from object areas which are adjacent in transverse x-direction by scanning the optical length of the reference arm of a two-beam interferometer. In the method described in the literature cited above, the measurement beam and reference beam always extend coaxially and centrally through the pupil of the eye. Therefore, it can only be used to acquire the geometry of the fundus, but not for partial length topography of the entire eye. Further, measurement errors are caused by transverse misalignments transverse to the axis of the eye during signal registration.
Another optical method which is suitable for this purpose is described in J. A. Izatt, M. R. Hee, D. Huang, J. G. Fujimoto, E. A. Swanson, C. P. Lin, J. S. Schuman, C. A Puliafito, SPIE Proc., 1877 (1993): 136-144. This relates to the method of optical coherence tomography (OCT). However, this method fundamentally suffers from the problem that eye movements during signal registration lead to errors in the measured structure. In particular, longitudinal movements in direction of the axis of the eye cause a falsification of the depth position or z-position of the measured structures.
Therefore, it is the primary object of the invention to provide arrangements for coherence topography of the eye by means of a series of depth signals which are measured by means of short coherence interferometry in different pupil points by scanning the optical length of the reference arm of a two-beam interferometer, wherein longitudinal movements in direction of the axis of the eye and transverse movements transverse to the axis of the eye do not cause a falsifying of the positions of the measured structures, and longitudinal depth signals or z-signals can be obtained at selected points in the pupil of the eye also outside of the visual axis.
This object is met in that the measurement beam of a short coherence interferometer is radiated into the pupil of the eye in a series of measurement positions and the reference beam, independent from the measurement beam, is fixedly directed to the corneal vertex and reflected at the latter. Every longitudinal movement of the eye then leads to the same phase displacement in the reference beam as in the measurement beam and has no effect on the short coherence interferometry. Further, the transverse position of the eye is monitored by means of a direction-dependent registration of the light reflected at the corneal vertex by means of a diode array or a four-quadrant diode and a criterion is obtained for the transverse alignment of the eye with respect to the beam axis. Transverse misalignments can be detected and compensated in this way. Finally, a pair of deflecting mirrors whose axes of rotation are oriented normal to one another is used for controlling the measurement beam at selected pupil points.
In the following, the invention will be described with reference to the figures.